I'm trying to simulate a long chain of atoms where I restrict the bond (length between two consecutive atoms) and angle (between three consecutive atoms). Therefore, if I want to simulate N particles, then I have N+3 degrees of freedom. The only interactions between atoms is due to Lennard-Jones Potential. I am using the classical Runge-Kutta fourth order method to integrate the equations. But, I got that the energy is always decreasing and then it keeps constant, like in the picture shown below. http://img194.imageshack.us/img194/3757/examplexy.png [Broken] What can I do to keep constant the energy ? What is happening ? I'm new with the topic. My english is not that good, so if you do not understand something, just let me know it. Thanks !